Selenium rectifier



Patented Aug.- 3, 1948 SELENIUM RECTIFIER Stanley S. Fry, North Chicago, 111., assignor to Fansteel Metallurgical Corporation, North Chicage, 11]., a corporation of New York No Drawing. Application November 11, 1944, Serial No. 563,100

7 Claims. (Cl. 175-366) This invention relates to blocking layer devices such as selenium rectifiers and light sensitive devices, and more specifically to means for forming an artificial blocking layer on the selenium.

In the production of blocking layer devices having a semi-conductor layer, for example, selenium rectifiers, a rigid carrier electrode or supporting electrode is provided with a thin coating or layer of selenium. The supporting electrode may be formed of nickel, nickel-plated iron, aluminum, magnesium, beryllium or other metals and alloys. A common practice in the production of selenium rectifiers includes grit blasting the iron disc or plate and electroplating the disc with nickel. The purpose of the grit blasting is to present a roughened surface to the selenium and to thereby improve the adherence of the selenium to the carrier electrode. The disc is then thoroughly cleaned and a thin film of selenium is applied over the nickel layer.

The selenium film or layer may be formed by a variety of methods. The plate or disc may be heated to a temperature above the melting point of selenium, for example, to a temperature of from 230 to 250 C., and the selenium in stick form may be rubbed across the heated plate in film. Another method of application includes placing a measured quantity of powdered selenium or selenium in pellet form on the heated plate and flowing the melted selenium over the surface. The melted selenium is usually spread over the heated plate by mechanical means, as with a glass rod. .The selenium may also be deposited on the carrier electrode from a vapor phase. The vaporization method is commonly employed in depositing the selenium film upon the light metal carrier electrodes. Various materials may be added to the selenium to increase its conductivity and otherwise impart desired characteristics and properties.

The selenium is then transformed into its gray crystalline state by heat treatment. The coated discs are generally stacked with mica, aluminum or other inert, smooth-surfaced discs or washers in contact with the selenium and between adjacent plates, and the stack subjected to a moderate pressure. then subjected to a relatively low temperature, that is, at a temperature below 150 C., and maintained at such temperature for from one hour to four or five hours. During this stage of the heat treatment the selenium softens so that the pressure produces a layer of selenium of relatively uniform thickness and having a smooth order to melt the selenium and form the desired surface.

The stacks are withdrawn from the oven or heat treating furnace and the platesare removed from the stack. The plates are then given a further heat treatment at a temperature approaching the melting point of selenium. This heat treatment is generally conducted at temperatures between about 200 C. and the melting point of selenium, for example, about 210 C.,'

for a period of from fifteen minutes to several hours. a layer of selenium is produced having a smooth surface of more or less uniform thickness and the selenium is transformed from its amorphous, non-conducting form into its gray, crystalline, conducting form.

The smooth surface of the selenium film is then treated to form an artificial blocking layer and a counter-electrode consisting of a relatively low melting point alloy is applied, as by spraying, over the selenium coating.

The final step in the manufacture of the rectifier plates consists of an electrical forming treatment., This treatment consists of subjecting the plate to either an alternating or direct current until a high resistance is developed in the reverse direction. This step may consist of applying to the plate in the reverse direction a direct current voltage below about 15 volts or a pulsating direct current starting with a low voltage of about 8 volts and gradually increasing the voltage to about 21 volts over a period of several hours.

In the use of an alternating current, it is necessary to include in the circuit a current limiting resistor because one-half cycle flows in the forward direction of the rectifier plate. The voltage may be about 20 volts and the period required for forming is generally greater than that required when a direct current is employed because 1 tion and an artificial blocking layer formed on the The stacks under pressure are surface of the selenium. A second electrode is selenium. This electrode may consist of a lightpermeable film of metal or a metal grid. The

application of the light-permeable metal may be This invention is directed to a method of form- During the combined heat treatment,-

A suping an artificial blocking layer on the film of selenium in the manufacture of blocking layer devices. This method is not limited to any specific form, type or composition of carrier electrode or counter-electrode. Any method may be employed in providing the carrier electrode with the film or layer of gray, crystalline, conducting selenium. The seleniumlayer may or may'not contain added materials to impart desired characteristics and properties.

The principal object of this invention is to provide a method of forming an artificial blocking layer on the surface of the selenium layer.

Other objects and advantages of this invention will become apparent from the following description and claims.

The present invention contemplates subjecting the surface of the selenium layer in its gray,

crystalline modification to the vapors resulting from heating a-benzildi'oxime in the atmosphere to'its melting or decomposition temperature which is about 237 C.

The specific details of the manufacturing process of the element are dependent upon the type of unit being made and the class of service for which it is designed. For purposes of illustration, the production of selenium rectifier plates is described.

A carrier electrode consisting of a grit blasted, nickel-plated iron disc is provided with a layer or film of selenium in its gray; crystalline form by any of the aforementioned methods. The selenium may contain added materials to improve the conductivity, as is well known to the art. The selenium surface is then exposed to the hot vapors resulting'from heating of m-bBIlZildlOXlIlli-Z to its decomposition temperature. The period of exposure of the selenium surface to the vapors may vary from a few seconds to several minutes,

I prefer to expose the-plate to these vapors for from about five seconds to about fifteen seconds.

The a-b'enzildioxime may be placed in an open container and heated to the required temperature. The plates are exposed individually to the vapors by manually or mechanically passing each plate over the open top of the container through the rising vapors. The plates may be placed upon an endless conveyor belt which passes through a chamber having an atmosphere of the vapors.

The counter-electrode is then applied as by spraying a low melting point alloy over the treated selenium surface. The rectifier plate is finally subjected to any of the desired electrical forming processes known to the art.

One of the methods commonly employed in grading and in evaluating the quality of selenium rectifier plates is the measurement of the current passed by the plate in the reverse or blocking direction upon the application of a direct current voltage to the plate in the blocking direction. An ideal or theoretically-perfect rectifier plate would have no leakage current fiow under such condition.

In order to compare the current leakage of rectifier plates made in accordancewith my inven-' tion and rectifier'plates made without the artificial blocking layer, a group of nickel-plated iron discs, having an area of about 12 sq. cm., was provided with a layer of selenium in its gray, crystalline form. The discs were heated to a temperature or about 240 C. and selenium in stick form rubbed across the plate. The selenium was spread over the surface" of the heated discs with a glass'rod. The plates were then stacked with mice discs covering the selenium layer and 4 the stack placed under pressure. The stack was heated to a temperature below 150 C. for about 2 hours. The pressure was then removed and the plates heated to about 210 C. and maintained at this temperature for about V2 hour. The group of plates was then divided into two series.

The discs of one series of plates were individually exposed for about 10 seconds to vapors formed by heating u-benzildioxime in an open vessel to its decomposition temperature. The discs of the other series were used as standard or control plates.

The discs of both series of plates were subsequently provided with a counterelectrode and subjected to the same electrical forming treatment.

A rectified direct current voltage of 23 volts was applied in the reverse or blocking direction to the individual discs of each series. The leakage current or the reverse current flow in the standard or control plates averaged approximately milliamperes. The leakage current in the plates made in accordance with the present invention averaged approximately 20 milliamperes.

The improvement in the operating characteristics of rectifier plates made in accordance with this invention is quite apparent from this data. It is obvious that the numerical values of the leakage current will differ with different size rectifier plates and also with selenium having various addition agents. The data is, however, representative of the improvements attained in the practice of my invention.

I claim:

1. The method of producing blocking layer devices which comprises providing a supporting electrode with a layer of selenium in its gray, crystalline form, subjecting the surface of the selenium layer to the action of vapors formed by heating a-benzildioxime to its decomposition temperature and applying a counter-electrode over the treated selenium surface.

2. The method of producing blocking layer devices which comprises applying a layer of selenium to a supporting electrode, transforming the selenium into its gray, crystalline form, subjecting the surface of the selenium layer to the action of vapors formed by heating a-benzildioxlme to its decomposition temperature and applying a counter-electrode over the treated selenium surface.

3. The method of producing blocking layer devices which comprises applying a layer of selenium to a supporting electrode, transforming the selenium into its gray, crystalline form, exposing the surface of the selenium layer for from about five seconds to about fifteen seconds to the vapors formed by heating a-benzildioxime to its decomposition temperature and applying a counter-electrode over the treated selenium surface.

4. The method of producing selenium rectifier plates which comprises providing a supporting electrode with a layer of gray, crystalline selenium, subjecting the surface of the selenium layer to the action of vapors formed by heating a-benzildioxime to its decomposition temperature, applying a counter-electrode over the treated selenium surface and subjecting the composite unit to an electrical forming operation.

5. The method of producing selenium rectifier plates which comprises applying a layer of selenium to a supporting electrode, transforming the selenium into its 'gray, crystalline form, passing the coated plate through the vapors age-cases formed by heating a-benzildioxime to its dedioxime to its decomposition temperature and a w counter-electrode on the treated selenium surface.

'7. A selenium rectifier plate comprising a supporting electrode, a layer of selenium on the electrode, the surface of the selenium being 15 treated with vapors formed by heating a-benzildioxime to its decomposition temperature and a counter-electrode over the treated selenium surface.

STANLEY S. FRY.

REFERENCES EK'IED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,121,603 Lotz June 21, 1938 2,193,598 Lotz Mar. 12, 1940 2,328,440 Esseling et a1 Aug. 31, 1943 2,362,545 Ellis et a1 Nov. 14, 1944 2,422,192 Fry June 17, 1947 

